Create breaking-change-proposal.md issue template (#6892)

* Create breaking-change-proposal.md issue template

* Update breaking-change-proposal.md

.github/ISSUE_TEMPLATE/breaking-change-proposal.md

@@ -0,0 +1,27 @@
+---
+name: Breaking change proposal
+about: Track a breaking change in Haystack
+title: ''
+labels: breaking change
+assignees: ''
+
+---
+
+## Summary and motivation
+
+Briefly explain how the change is breaking and why it is needed.
+
+## Checklist
+
+```[tasklist]
+### Tasks
+- [ ] The changes are merged in the `main` branch (Code + Docstrings)
+- [ ] Release notes have documented the breaking change
+- [ ] A new version of `haystack-ai` has been released on PyPI
+- [ ] Docs at https://docs.haystack.deepset.ai/ were updated
+- [ ] Integrations on [haystack-core-integrations](https://github.com/deepset-ai/haystack-core-integrations) were updated (if needed) - This step might require a [Breaking change proposal](https://github.com/deepset-ai/haystack-core-integrations/issues/new?assignees=&labels=breaking+change&projects=&template=breaking-change-proposal.md&title=) on the repo
+- [ ] Notebooks on https://github.com/deepset-ai/haystack-cookbook were updated (if needed)
+- [ ] Tutorials on https://github.com/deepset-ai/haystack-tutorials were updated (if needed)
+- [ ] Articles on https://github.com/deepset-ai/haystack-home/tree/main/content were updated (if needed)
+- [ ] Integration tile on https://github.com/deepset-ai/haystack-integrations was updated (if needed)
+```